Restoring optical pulses

ABSTRACT

In one embodiment of the present invention, a method includes introducing an optical signal into a first arm and a second arm of an optical device; self-phase modulating the optical signal propagating in the first arm; and outputting a high intensity portion of the optical signal spatially separated from a low intensity portion of the optical signal. In such manner, optical signals input into the optical device may be restored via cleaning and shaping.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/370,432, filed on Feb. 20, 2003.

BACKGROUND

The present invention relates to optical communications and morespecifically to the restoring of optical pulses to remove distortion.

In optical communication networks, optical pulses carrying informationbits undergo distortion for several reasons. First, dispersion in anoptical fiber causes the pulse to spread temporally. Second, non-uniformamplification in optical amplifiers creates wings and humps in thepulse. Third, optical components in a network may have irregulartransfer functions. Pulses may also be distorted in other ways.Distortion such as unwanted low intensity wings, which are added to highintensity pulse streams, degrade performance of an optical system andlimit either data transmission bit-rate or network link length.

Such distortion is undesired as typical optical pulse widths ordurations in optical communication networks are extremely short. Forexample, in optical carrier (OC)-48 systems transmitting data at a rateof 2.5 Gigabits per second (GB/s), the pulse width is about 400picoseconds (ps). In OC-192 systems (10 GB/s), the pulse width is about100 ps, and in OC-768 systems (40 GB/s), the pulse width is about 25 ps.Thus, higher data rates require shorter optical pulses, which suffergreater degradation in the time domain due to dispersion.

Presently, repeaters are provided along a network that acts astransceivers to convert optical pulses to electrical signals, restorethe pulses by cleaning undesired artifacts and reshaping, amplify, andthen retransmit them as optical pulses. In a metropolitan network suchrepeaters may be placed every few hundred meters to several kilometers(km) apart, whereas in a long-haul network, such repeaters may be placedevery few kilometers to tens of kilometers. However, such repeatersraise network costs and complexity and do not fully remove distortionfrom the optical signals. In a wavelength division multiplexed (WDM)network system employing multiple wavelength channels, such pulseregeneration becomes very expensive since the individual channels mustfirst be spatially separated using a demultiplexer, pulses restored, andchannels recombined using a multiplexer.

A need thus exists to remove unwanted distortion from optical pulses andrestore optical pulses by cleaning and shaping them to remove suchdistortion without the above drawbacks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a planar integrated optical device inaccordance with one embodiment of the present invention.

FIG. 1B is a schematic diagram of a planar integrated optical device inaccordance with a second embodiment of the present invention.

FIG. 2A is a graphical representation of a typical pulse shape emittedby an optical transmitter.

FIG. 2B is a graphical representation of a typical pulse shape distortedafter propagating in an optical fiber.

FIG. 2C is a graphical representation of a restored optical pulse inaccordance with one embodiment of the present invention.

FIG. 2D is a graphical representation of low intensity wings separatedfrom a restored optical pulse in accordance with one embodiment of thepresent invention.

FIG. 3 is a block diagram of an optical network in accordance with oneembodiment of the present invention.

FIG. 4 is a cross-section view of a waveguide structure in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION

In one embodiment, the present invention includes an optical device thatrestores distorted optical pulses by cleaning and shaping them in thetime domain. Referring now to FIG. 1A, shown is a schematic diagram ofan optical device in accordance with one embodiment of the presentinvention. Optical device 10 may be based on a Mach-Zehnderinterferometer (MZI) configuration, and may be a planar lightwavecircuit (PLC) in one embodiment.

As shown in FIG. 1A, optical device 10 may be formed on a substrate 20,which may be for example silicon, crystals such as Lithium Niobate(LiNbO3), III-V semiconductors such as Indium Phosphide (InP) and IndiumGallium Arsenide (InGaAs), polymer materials such as polyimide, oranother substrate material. Optical device 10 may include a firstwaveguide 25 which may receive incoming optical pulses, such as inputpulse 15. The incoming optical pulse 15 may be equally split by adirectional coupler 35, which in one embodiment may be a 3 dB coupler. Afirst arm 40 of the device 10 may be formed of a third order nonlinearoptical (NLO) medium. Such an NLO medium may have a refractive index naccording to the equation:n=n ₀ +n ₂ I  [1]where n₀ equals an intensity independent portion of the refractiveindex, n₂ is the nonlinear index, and I is the instantaneous opticalintensity.

In one embodiment, the NLO medium or material may have largehyperpolarizability. Such material, in one embodiment, may bepolydiacetylene-para-toluene-sulfonate (PDA-PTS). Such material may bedesirable as it has an exceptionally high nonlinear refractive index dueto a one dimensional confinement of delocalized π-electrons in thepolymer chain. Such PDA-PTS material may have a nonlinear index ofapproximately 10⁻⁵ centimeter² per Megawatt (cm²/MW). Such a nonlinearindex is approximately 5 orders of magnitude larger than that of silica.However, in other embodiments, NLO materials may have a larger orsmaller nonlinear index. In other such embodiments, other polymers, suchas poly-p-phenylene vinylene (PPV), polyacetylene, polythiophene, orpoly-indenofluorene (PIF), may be used as the NLO material. In stillother embodiments, III-V semiconductors such as Indium Phosphide (InP)or Indium Gallium Arsenide (InGaAs) may be used as the NLO material. Instill other embodiments, a linear medium such as silica doped with anonlinear medium or a combination of several nonlinear materials may beused.

In various embodiments, the optical pulse propagating through first arm(or “NLO arm”) 40 may undergo self-phase modulation wherein thelight-induced phase change due to the intensity-dependent refractiveindex of the NLO medium is given by the equation: $\begin{matrix}{{\Delta\phi} = {\frac{2\pi}{\lambda}n_{2}{Id}}} & \lbrack 2\rbrack\end{matrix}$where n₂ is the nonlinear index, d is the length of the NLO arm 40, I isthe instantaneous optical intensity and λ is the wavelength. In variousembodiments, the length of first arm 40 may be between approximately oneto twenty centimeters (cm). The length of the nonlinear arm for aparticular material system and application may depend on the magnitudeof the nonlinear index of the material and optical intensity used in theapplication.

In certain embodiments, the parameters of NLO arm 40 may be designed sothat a central high-intensity portion of the optical pulse undergoesself-induced π phase retardation. The second portion of the opticalpulse separated by directional coupler 35 travels through a second arm50. This second arm 50 may be made of silica, in one embodiment. Asshown in FIG. 1A, the two portions of the optical pulse may berecombined at a second directional coupler 55, which in one embodimentmay be a 3 db coupler.

After such recombining, a high intensity main portion of the pulse mayexit device 10 via a first waveguide 60, while low intensity wings ofthe pulse may exit device 10 via a second waveguide 70. It is to beunderstood that in various embodiments, the low intensity wings may beundesired and not used further. In such manner, the principal portion ofthe pulse may be spatially separated from the undesired wings withoutrequiring any external electrical power.

While described above as a symmetric MZI structure, in otherembodiments, a device in accordance with the present invention may beformed of an asymmetric MZI structure. In a conventional asymmetric MZIstructure, the two arms of the device may have a π phase difference.Thus, in an embodiment of FIG. 1A using an asymmetric MZI structure, thehigh intensity main portion of the pulse may exit device 10 via secondwaveguide 70 and the low intensity wings may exit via first waveguide60. In other words, in an asymmetric configuration, the high and lowintensity portions may exit from the opposite waveguide than they wouldin a symmetric configuration.

In other embodiments, a device in accordance with the present inventionmay be formed using a push-pull MZI structure. In such a push-pullconfiguration, both arms of the device may alter the phase of anincoming signal by a phase of π/2. Accordingly, the resulting phasedifference of the two arms may be a π phase difference. Thus in anembodiment of the present invention using such a push-pull MZIconfiguration, one arm may be composed of a positive nonlinear materialand a second arm may be composed of a negative nonlinear material. Thusthe combination of the two arms may provide a phase difference of π. Useof such a push-pull configuration may be desirable in certainembodiments to reduce the length of first and second arms. Inembodiments in which the arms have positive and negative π/2 phaseshifts, the arms may have a length of between approximately 0.5 cm to 10cm.

In yet other embodiments, a device in accordance with the presentinvention may be formed using a Michelson type interferometerconfiguration. Referring now to FIG. 1B, shown is a schematic diagram ofa device in accordance with a second embodiment of the present inventionbased on a Michelson type interferometer configuration. The use of thesame reference numerals as in FIG. 1A indicate similar components. Asshown in FIG. 1B, incoming pulse 15 passes through a circulator 30 andinto waveguide 25, and is then separated by coupler 35 and fed into NLOarm 40 and second arm 50. As discussed above, NLO arm 40 causes theportion of the optical pulse propagating therethrough to undergoself-phase modulation. The two portions of the optical pulse may bereflected by a reflective facet 45 which may be, for example, adielectric or metallic mirrored surface. Then the two portions of theoptical pulse may be recombined at coupler 35.

After such recombining, the high intensity main portion of the pulse mayexit the device via waveguide 25, where it may then pass throughcirculator 30 and exit via an optical fiber 22, for example. While shownas a separate component in the embodiment of FIG. 1B, in otherembodiments a circulator may be formed on the same substrate as a devicein accordance with the present invention.

In the embodiment of FIG. 1B, the low intensity portion of the pulse mayexit the device via waveguide 28. As discussed above, the low intensityportion may be unused in further processing. While discussed as asymmetric Michelson type interferometer, in other embodiments anasymmetric Michelson type interferometer may be used.

In still other embodiments, devices may be formed using optical fibersrather than waveguide structures. In such embodiments, one arm may beformed using an optical fiber doped with a NLO material. In certainembodiments, such a doped optical fiber may have a nonlinear index onthe order of approximately 10⁻⁸ to 10⁻⁶ cm²/MW. In certain embodiments,the optical fibers may have a length of between approximately one meterto twenty meters.

Embodiments of the present invention may be used in optical networksaccommodating multiple channels, as the device may be transparent to thenumber of channels, and incoming pulses need not be demultiplexed.Restoring optical pulses in accordance with embodiments of the presentinvention may be performed rapidly using ultrafast nonlinear opticalprocesses, requiring only femtoseconds for operation.

In certain embodiments, the distance between a transmitter and areceiver may be increased, as a device in accordance with the presentinvention may desirably restore optical pulses degraded duringtransmission over optical links of extended lengths. More so,embodiments of the present invention may be incorporated in an opticalnetwork to reduce the need for repeaters, or to extend the lengthbetween repeaters. In certain embodiments, a receiver and transmittermay be located at extended distances of between approximately 10 km and500 km, and degraded optical pulses may be cleaned and shaped inaccordance with embodiments of the present invention.

In certain embodiments, a device in accordance with an embodiment of thepresent invention may be integrated in a PLC on a single substrate alongwith other active and passive optical components such as an arrayedwaveguide grating (AWG), a variable optical attenuator (VOA), a lasersource, and the like.

Referring now to FIGS. 2A-2D, shown are graphical representations ofvarious optical pulses. FIG. 2A is a graphical representation of atypical pulse shape emitted by an optical transmitter. FIG. 2B is agraphical representation of the pulse shape of FIG. 2A after distortionby propagation in an optical fiber. FIG. 2C is a cleaned and shapedversion of the optical pulse of FIG. 2A after being restored inaccordance with one embodiment of the present invention. In one suchembodiment, the pulse shape of FIG. 2C may be the high intensity opticalpulse exiting device 10 via waveguide 60. FIG. 2D is a graphicalrepresentation of low intensity wings which exit device 10 via waveguide70 in one embodiment of the present invention.

Referring now to FIG. 3, shown is a block diagram of an optical network100 in accordance with one embodiment of the present invention. As shownin FIG. 3, a plurality of lasers 105 ₁-105 _(n) may be coupled to aplurality of waveguides 110 ₁-110 _(n) and provide optical pulses to amultiplexer 120. Multiplexer 120 multiplexes the multiple optical pulsesonto a single optical fiber 125. In one embodiment, the optical pulsesmay be multiplexed using wavelength division multiplexing (WDM). In suchmanner, a number of data channels having different wavelengths may becarried on a single optical fiber.

In one embodiment, optical fiber 125 may be coupled to an amplifier 130,which in turn may be coupled to another optical fiber 125. Optical fiber125 may be coupled to an optical cross connect 140. Optical crossconnect 140 may be used to route certain channels from the opticalpulses to other desired network locations, as shown by drop ports 147 ₁and 147 ₂. Further, optical cross connect 140 may be used to injectincoming optical pulses into optical network 100 via add ports 145 ₁ and145 ₂.

The optical pulses may then travel on optical fiber 125, which iscoupled to a nonlinear Mach-Zehnder interferometer (NLMZI) device 150 inaccordance with an embodiment of the present invention. As discussedabove, device 150 may be used to restore optical pulses by cleaning andshaping them such that exiting pulses are restored to a higher intensityand an undistorted shape.

In one embodiment, NLMZI device 150 may be coupled to a demultiplexer160 which separates the optical pulses into a plurality of waveguides165 ₁-165 _(n), each corresponding to a different wavelength (in a WDMnetwork). In turn, waveguides 165 ₁-165 _(n) may be coupled torespective photodetecters 170 ₁-170 _(n), which may be used to convertthe optical pulses into electrical signals for further processing anduse.

As discussed above, in one embodiment, the waveguide structures may bemanufactured on a silica-on-silicon platform. Referring now to FIG. 4,shown is a cross-section view of a waveguide structure in accordancewith one embodiment of the present invention.

In one embodiment, the waveguide structure may be formed as follows. Afirst layer 210 may be formed on a substrate 200. In one embodiment,approximately a 15 micron (um) thick thermal oxide (SiO₂) layer may begrown on a silicon substrate. This first layer 210 may serve as thelower clad of the waveguide. Next, a core layer 220 may be formed onfirst layer 210. In one embodiment, the core layer 220 may be formed bya plasma-enhanced chemical vapor deposition (PECVD) process. In variousembodiments, core layer 220 may be about 6 micron thick and may begermanium-doped oxide (Ge—SiO₂). Next, core layer 220 may be patternedto form the waveguide shape. In one embodiment, a waveguideapproximately 6 micron by 6 micron may be patterned via conventionalphotolithography and etching processes. Then, an upper layer 230 may bedeposited. In one embodiment, such an upper clad may be formed by aPECVD process, and may be approximately 15 micron thick and formed ofboron-phosphorus-doped oxide (BP—SiO₂) material. In certain embodiments,an oxide layer (not shown in FIG. 4), such as a 1 micron thick undopedoxide, may be grown above upper layer 230 to protect it fromenvironmental degradation.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: introducing an optical signal into a first armand a second arm of an optical device; self-phase modulating the opticalsignal propagating in the first arm; and outputting a high intensityportion of the optical signal spatially separated from a low intensityportion of the optical signal.
 2. The method of claim 1, wherein theself-phase modulating comprises π-phase retarding the optical signalpropagating in the first arm.
 3. The method of claim 1, furthercomprising outputting the high intensity portion of the optical signalon a first waveguide.
 4. The method of claim 1, further comprising usinga nonlinear optical medium of the first arm to perform the self-phasemodulating.
 5. The method of claim 1, further comprising equallyseparating the optical signal into a first portion and a second portionprior to introduction into the first arm and the second arm.
 6. A methodcomprising: separating an optical pulse into a first pulse portionpropagating through a first arm of an optical device and a second pulseportion propagating through a second arm of the optical device;self-phase modulating the first pulse portion in the first arm;combining the first pulse portion and the second pulse portion into anoutput optical signal; and outputting a high intensity portion of theoutput optical signal spatially separated from a low intensity portionof the output optical signal.
 7. The method of claim 6, wherein theself-phase modulating comprises π-phase retarding the first pulseportion.
 8. The method of claim 6, further comprising outputting thehigh intensity portion on a first waveguide.
 9. The method of claim 6,further comprising using a nonlinear optical medium of the first arm toperform the self-phase modulating.